1. Field of the Invention
The present invention relates to a charged-particle beam writing apparatus and a charged-particle writing method, and more particularly to a charged-particle beam writing apparatus and charged-particle beam writing method in which a charged-particle beam is deflected by a main deflector and a sub deflector, which are disposed on the optical path of the charged-particle beam, and a predetermined pattern is written onto a sample located on a stage.
2. Background Art
With increasing integration densities of semiconductor integrated circuits, patterns of LSI (large scale integration) tend to be miniaturized and complex. Due to the miniaturization and complexity, a pattern is written directly onto a sample by means of a charged-particle beam such as an electron beam in place of exposure of the sample to light. In this technique, however, the writing speed and throughput are low compared with those obtained in a one-shot exposure method. Thus, attempts have been made to improve the throughput using various techniques.
In one of the techniques, the number of exposures is reduced by using a variable shaped beam instead of an electron beam having a circular cross section. In another one of the techniques, the stage moves continuously, not in accordance with a step and repeat method. In still another one, an electron beam is deflected according to a vector scanning method.
The combination of the abovementioned techniques makes it possible to greatly improve the throughput. In the vector scanning method, however, the deflection angle of an electron beam to be directed to a subfield varies depending on the position of the subfield within a frame. Due to the variation, the accuracy of the shape, position and size of a writing pattern may be degraded. Japanese patent laid-open No. 284392/1998 (Hei 10-284392) discloses a method for detecting a variation (occurring depending on the position of a spot of an electron beam deflected by a main deflector) in sensitivity of a sub deflector before formation of a pattern and correcting the detected value for the formation of the pattern.
It is necessary that an electron beam be deflected at a high speed and with high accuracy to improve the throughput. In order to drive a deflector by means of a deflection amplifier, however, a period of time (settling time) for settling an output voltage corresponding to a load for driving the deflector is required. In other words, a predetermined settling time is required to settle the output voltage in order to set the spot of a deflected electron beam to a target position. In addition, when the surface of a sample is irradiated with an electron beam during the settling time, a pattern written onto the sample is adversely affected. It is therefore necessary that a blanking mechanism be operated during the settling time to prevent the sample from being irradiated with the electron beam.
It is desirable that the time for starting an operation of a blanking mechanism (hereinafter referred to as the blanking start time) be identical to the timing for applying a voltage to a sub deflector when changing a position to be irradiated with the charged-particle beam. Thus, while the waveform of the output voltage is confirmed by an oscilloscope, the blanking start time is determined. In this method, however, there may occur adjustment errors on an individual basis. When the voltage is applied to the sub deflector before the blanking start time, an unnecessary region may be irradiated with the electron beam. Therefore, the accuracy of writing may be reduced. In a conventional technique, the voltage is applied to the sub deflector after a predetermined time elapses from the blanking start time.
The settling time is identical to a period of time for blanking. Thus, when the voltage is applied to the sub deflector after a predetermined time elapses from the blanking start time, the substantial settling time is reduced. This may result in the fact that formation of a pattern is started before output of the sub deflector is stabilized. Especially when the movement distance of the electron-beam irradiation position is large, the voltage to be applied to the sub deflector is large. It therefore takes a certain time to stabilize the output of the sub deflector. In this case, there is a higher possibility that formation of a pattern is started before the output of the sub deflector is stabilized. If the formation is started after the output of the sub deflector is stabilized, however, the settling time becomes longer, and the throughput is reduced.